Gratingly cliched plot and uncomfortable dialogue prevent Mama's Boy from becoming the amusing comedy it could have been. With a group of actors amazingly typecast: Jon Heder as the self-deluded social outcast, Diane Keaton as the overbearing mother trying to do what's best for her child and Anna Faris as the quirky pothead, the outcome should have been laughably mediocre yet drags from scene to scene to scene to scene . . .